Middle value selection circuit

ABSTRACT

The middle value selection circuit includes three high value selection circuits and one low value selection circuit. Each high value selection circuit consists of a pair of first NPN transistors and a first constant current circuit. The emitter of each first NPN transistor of the high value selection circuit is connected to the first constant current circuit and the base of the second NPN transistor of the low value selection circuit. The base of this second NPN transistor is connected to the second constant current circuit and to an output terminal. The collector of the second NPN transistor is connected to its base. The first constant current circuit produces a current which is twice the output current of the second constant current circuit. Two analog signals having mutually different combinations among three input analog signals are applied to the NPN transistors of one high value selection circuit. The analog signal having a greater value of the two analog signals is generated at the junction of the emitters of these NPN transistors. The analog signals, each representative of the greater value, from the high value selection circuits are applied the emitter of the second NPN transistor. The low value selection circuit produces an analog signal representative of the minimal value among the analog signals applied to the three second NPN transistors.

BACKGROUND OF THE INVENTION

This invention relates to a middle value selection circuit. Moreparticularly, the present invention relates to a middle value selectioncircuit which is particularly suitable for use in a multiple redundantcontrol apparatus for various systems for which high reliability is arequisite.

To improve the safety of a nuclear power plant, a triple redundantcontrol apparatus is used for an especially important control system.The middle value of the output signals from the three processing unitsis selected by one middle value selection circuit and is used as acontrol signal for controlling an actuator that is disposed in a controlvalve or the like.

The triple redundant control apparatus of the type described aboveincludes the middle value selection circuit or circuits in order toselect the middle value of the input and output signals, as describedabove. An example of the middle value selection circuit is disclosed inJapanese Patent Laid-Open No. 11638/1975 and Japanese Utility ModelLaid-Open No. 117001/1980.

The middle value selection circuit must select a signal which is themost approximate to a true value from the three signals; hence, it isnecessary to reduce the error of the output signals as much as possible.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a middlevalue selection circuit which is simple in construction and yet canprovide high accuracy.

The characterizing feature of the present invention resides in thatthree first selection means for receiving two analog signals of mutuallydifferent combinations among three input analog signals and producingeither one of the two signals and second selection means for selectingand producing one of the output signals of each of the first selectionmeans are composed of complementary semiconductor devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the conventional middle value selectioncircuit;

FIG. 2 is a block diagram of the triple redundant control system;

FIG. 3 is a circuit diagram of the middle value selection circuit inaccordance with one preferred embodiment of the present invention;

FIG. 4 is a circuit diagram of the constant current circuits applied tothe circuit shown in FIG. 3;

FIG. 5 is a diagram useful for explaining the operation of the middlevalue selection circuit shown in FIG. 3;

FIG. 6 is a circuit diagram of the middle value selection circuit inaccordance with another embodiment of the present invention;

FIG. 7 is a circuit diagram of the constant current circuits applied tothe circuit shown in FIG. 6;

FIGS. 8, 9, 11, 13, 14, 15, 16 and 17 are circuit diagrams of the middlevalue selection circuits in accordance with other embodiments of thepresent invention;

FIG. 10 is a circuit diagram of the constant current circuit applied tothe embodiments shown in FIGS. 9, 13 and 15; and

FIG. 12 is a circuit diagram of the constant current circuit applied tothe embodiments shown in FIGS. 11, 14 and 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based upon the clarification of the cause forthe occurrence of the error of the output signals in the conventionalmiddle value selection circuits. The results of the investigation willbe briefly described as follows.

The middle value selection circuit shown in FIG. 1 is disclosed in theaforementioned Japanese Patent Laid-Open No. 11638/1975.

The middle value selection circuit 1 consists of three PNP transistors2, 3, 4, three current limiting base series resistors 5, 6, 7 and threeprotective collector series resistors 8, 9, 10. Three input signals 11,12, 13 are applied to the three junctions between the base seriesresistors and the collector series resistors of the PNP transistors6-10, 8-7, 5-9, respectively. A signal corresponding to the middle valueof the three input signals is picked out from a connecting line 14 thatconnects the emitters of these PNP transistors.

It will be assumed that the input signals have the relation of voltageE₀ <E₁ <E₂ with E₀, E₁ and E₂ representing the input signals 11, 12 and13, respectively. Under this state, the PNP transistor 2 is cut off, thePNP transistor 3 is reversely conductive and the PNP transistor 4 isforwardly conductive. Accordingly, a voltage which is higher by thevoltage drop across the resistor 7 and the voltage drop across the baseand emitter of the PNP transistor 4 than E₁ as the middle value signalis generated on the connecting line 14. This middle value selectioncircuit produces the signal of the middle value of the three outputsignals while containing the error. The error contained in the outputsignal of the middle value selection circuit arises from theconstruction of the middle value selection circuit itself.

The inventors of the present invention have examined various methods ofreducing the error of the output signal of the middle value selectioncircuit and have found out that the error of the output signal of themiddle selection circuit can be reduced by constructing high and lowvalue selection circuits forming the middle value selection circuit insuch a manner that the polarity of the error of each selection circuitoutput signal becomes inverse. In other words, the inventors havereached the conclusion that the error can be reduced by constructing thehigh and low value selection circuits by complementary semiconductordevices. The embodiments of the present invention based upon thisfinding will be hereinafter explained.

One preferred embodiment of the present invention will be described withreference to FIGS. 2 and 3. FIG. 2 shows a triple redundant controlapparatus. In a nuclear power plant, three sensors 21, 22 and 23 aredisposed so as to detect the quantity of the same state in an importantcontrol system and equipments to be controlled are controlled on thebasis of the detection signals (analog signals) of these sensors. Thesensor output signals are applied to three middle value selectioncircuits 24A, 24B and 24C that are juxtaposed with one another. Themiddle value selection circuits 24A, 24B, 24C select the middle value ofthe output signals of the respective sensors. The output signal selectedby each middle value selection circuit is applied to a correspondingprocessing unit (controller) 25A, 25B, 25C. Each controller effectscalculation, comparison and judgement on the basis of the input signaland produces an output signal for controlling the equipment to becontrolled. The control signal generated from each controller is appliedto one middle value selection circuit 24D. This middle value selectioncircuit 24D selects the middle value of each control signal and producesan output signal to an actuator of the equipment to be controlled. Theactuator 26 controls the equipment to be controlled on the basis of thecontrol signal of the middle value. The triple redundant controlapparatus can control the equipment with the output signal, which is themost approximate to the true value, even if the output signal changes asone of the sensors 21, 22, 23 or one of the middle value selectioncircuits 24A, 24B, 24C or one of the controllers 25A, 25B, 25C becomesout of order.

FIG. 3 shows the definite construction of the middle value selectioncircuits 24A-24D. The middle value selection circuit 24 includes NPNtransistor 31A-31F, PNP transistors 32A, 32B and 32C, and constantcurrent circuits 33A, 33B, 33C and 33D. The middle value selectioncircuit 24 consists of the high value selection circuits 34A, 34B and34C and the low value selection circuit 35. The high value selectioncircuits 34A-34B are disposed at the prior stage with the low valueselection circuit 35 at the posterior stage.

The high value selection circuit 34A consists of a pair of NPNtransistors 31A, 31B and a constant current circuit 33A. The high valueselection circuit 34B consists of NPN transistors 31C, 31D and aconstant current circuit 33B. The high value selection circuit 34Cconsists of NPN transistors 31E, 31F and a constant current circuit 33C.The low value selection circuit 35 consists of PNP transistors 32A, 32B,32C and a constant current circuit 33D. The NPN transistor of the highvalue selection circuit is complementary to the PNP transistor of thelow value selection circuit. The NPN and PNP transistors of the high andlow value selection circuits in the later-appearing embodiments are alsocomplementary to one another.

Reference numerals 36A, 36B, 36C represent the input terminals of themiddle value selection circuit 24. The input terminal 36A is connectedto the base of the NPN transistor 31A and to the base of the NPNtransistor 31F by a line 43. The input terminal 36B is connected to thebase of the NPN transistor 31B and to the base of the NPN transistor 31Cby a line 44. The input terminal 36C is connected to the base of the NPNtransistor 31D and to the base of the NPN transistor 31E. The collectorsof the NPN transistors 31A through 31F are connected to a line 46. Thisline 46 is connected to the terminal 38. The emitters of the NPNtransistors 31A, 31B are connected to the constant current circuit 33Aby a line 47. A line 48 connects the emitters of the NPN transistors31C, 31D to the constant current circuit 33B. A line 49, which isconnected to the constant current circuit 33C, is connected to theemitters of the NPN transistors 31E, 31F. The constant current circuits33A through 33C are connected to a terminal 39 by a line 50.

A line 51 connects the line 47 to the base of the PNP transistor 32C. Aline 52 connects the base of the PNP transistor 32B to the line 48. Aline 53 connects the base of the PNP transistor 32A to the line 49. Theemitters of the PNP transistors 32A through 32C are connected to theoutput terminal 37 via a line 54. The emitters of the PNP transistors32A through 32C are connected to the constant current circuit 33D by aline 57. A line 56 connects the constant current circuit 33D to theterminal 38. On the other hand, the collectors of the PNP transistors32A through 32C are connected to the terminal 39 by a line 55.

FIG. 4 shows an example of the constant current circuit. This constantcurrent circuit is generally referred to as a "current mirror circuit".The constant current circuits 33A-33C consist of NPN transistors 68-70,respectively. The constant current circuit 33D consists of a PNPtransistor 71. A line 61 connects the terminal 38 to the emitter of thePNP transistor 58. A line 63 in which a resistor 60 is disposed connectsthe collector of the PNP transistor 58 to the collector of the NPNtransistor 59. A line 65 which is connected to the terminal 39 isconnected to the emitter of the NPN transistor 59. A line 62 connectsthe base of the PNP transistor 71 to the base of the PNP transistor 58.A line 66 connecting the base and collector of the PNP transistor 58connects also the line 62 to the line 63. A line 64 connected to thebase of the NPN transistor 59 is connected to the bases of the NPNtransistors 68 through 70, respectively.

The lines 63 and 64 are connected to each other by a line 67 thatconnects the base and collector of the NPN transistor 59 to each other.A positive d.c. current is applied to the terminal 38 while a negatived.c. current is applied to the terminal 39. The constant current circuitshown in FIG. 4 is arranged so that the current flowing through the PNPtransistor 58, the resistor 60 and the NPN transistor 59 flows alsothrough the PNP transistor 33D and the NPN transistors 33A through 33C.

The operation of the middle value selection circuit 24 of thisembodiment will be described. It will be assumed here that the signalsA, B and C (analog signals) applied to the respective input terminals36A, 36B and 36C have the voltages E₁, E₂ and E₃, respectively, andthese voltages have the relation E₁ >E₂ >E₃. The signal A of the voltageE₁ is applied to the bases of the NPN transistors 31A and 31F by theline 43. The signal B of the voltage E₂ is applied to the bases of theNPN transistors 31B and 31C by the line 44. The signal C of the voltageE₃ flows through the line 45 and reaches the bases of the NPNtransistors 31D and 31E. Since the relation E₁ >E₂ exists at the outputend which is the junction between the emitters of the NPN transistors31A, 31B of the high value selection circuit 34, a voltage which islower by the voltage V_(BE) of the NPN transistor 31A than the voltageE₁ (that is, E₁ -V_(BE)) is produced. This voltage signal flows throughthe lines 47 and 51 and is applied as the output of the high valueselection circuit 34A to the base of the PNP transistor 32C of the lowvalue selection circuit 35. The voltage V_(BE) is the baseemittervoltage of the transistor. The relation E₂ >E₃ exists between thevoltages E₂ and E₃ of the signals applied to the NPN transistors 31C and31D of the high value selection circuit 34B. Accordingly, a voltagewhich is lower by the voltage V_(BE) of the NPN transistor 31C than thevoltage E₂ (that is, E₂ -V_(BE)) is generated at the junction of theemitters of these transistors. This voltage signal (E₂ -V_(BE)) flowsthrough the lines 48 and 52 and is applied as the output of the highvalue selection circuit 34B to the base of the PNP transistor 32B of thelow value selection circuit 35. Furthermore, since the relation E₁ >E₃exists at the junction of the emitters of the NPN transistors 31E and31F of the high value selection circuit 34C, a voltage lower by thevoltage V_(BE) of the NPN transistor 31F than the voltage E₁ (that is,E₁ -V_(BE)) is generated and this voltage signal (E₁ -V_(BE)) flowsthrough the lines 49 and 53 and is applied to the base of the PNPtransistor 32A of the low value selection circuit 35 as the output ofthe high value selection circuit 34C.

Since the relation (E₁ -V_(BE) >(E₂ -V_(BE)) exists when the PNPtransistor 32A through 32C of the low value selection circuit 35 receivethe respective voltage signals, a voltage higher by V_(BE) of the PNPtransistor 32B than the voltage (E₂ -V_(BE)), [that is, (E₂-V_(BE))+V_(BE) =E₂ ], is generated at the junction of the emitters ofthese three transistors. This output signal is generated as the outputsignal of the middle value selection circuit 24 from the output terminal37.

The voltage V_(BE) is a kind of error. This error assumes a negativevalue in the high value selection circuits 34A-34C and a positive valuein the low value selection circuit 34D. More definitely, the errorsresult from the difference of the characteristics of the NPN and PNPtransistors. As the high and low value selection circuits having theinverse characteristics are combined as in this embodiment, the erroroccuring in the high value selection circuit is offset by the erroroccurring in the latter so that the error as the middle value selectioncircuit drops and hence, the accuracy of the middle value selectioncircuit can be improved as much. Especially because the constant currentcircuits 33A through 33D are respectively disposed for the high valueselection circuits 34A-34C and the low value selection circuit 35, theabsolute value of the voltage V_(BE) of the high value selection circuitbecomes equal to that of the low value selection circuit. Accordingly,the error of the output signal of the high value selection circuitvirtually cancels out the error of the output signal of the low valueselection circuit. At this time, the accuracy of the middle valueselection circuit becomes maximum. The middle value selection circuit 24generates a signal having the same value as the middle value of thethree input signals.

When the d.c. power voltage applied between the terminals 38 and 39changes, the current each of the constant current circuits of the highvalue selection circuits 34A-34C and low value selection circuit 35 alsochanges. However, the change value of the current of these constantcurrent circuits is the same and the absolute value of the change valueof the voltage V_(BE) resulting from each transistor becomes equal. Inthis case, too, the change of the voltage V_(BE) cancels out the outputsignals of the high and low value selection circuits, as describedpreviously. Since NPN transistors are used for the high value selectioncircuits, the output current of the constant current circuits can becompensated for by the current amplifying operation of the NPNtransistors even if the current capacity of the signals to be applied tothe terminals 36A, 36B and 36C is small.

The reason why the error of the high value selection circuit can becancelled by the error of the low value selection circuit will beexplained in detail. The base-emitter voltage V_(BE) of the transistoris expressed by the following equation: ##EQU1## where k is Boltzmann'sconstant (1.38×10⁻²³ J/K), q is the charge quantity of electron(1.6×10⁻¹⁹ Coulomb), T is absolute temperature, I_(c) is the collectorcurrent of the transistor and I_(s) is the collector junction reversesaturation current per emitter area of the transistor.

Since E₁ >E₂ >E₃, the output voltage E_(out) of the middle valueselection circuit 24 is expressed by the following equation:

    E.sub.out =E.sub.2 -V.sub.BEH +V.sub.BEL                   (2)

where V_(BEH) is the base-emitter voltage of the transistor of the highvalue selection circuit and V_(BEL) is the base-emitter voltage of thelow value selection circuit.

These base-emitter voltages V_(BEH) and V_(BEL) are expressed as followswith the proviso that the collector current I_(c) of each transistorbecomes equal to the output current I_(o) of the constant currentcircuit: ##EQU2##

From the relation (3) and (4), V_(BEH) =V_(BEL). When this relation issubstituted in equation (2), E_(out) =E₂. Accordingly, the differenceΔE_(out) between the input and output voltages of the middle valueselection circuit 24 is 0(=E_(out) -E₂).

FIG. 5 shows the relation between the three input signals E₁ (twodot-and-chain line), E₂ (dash line), E₃ (dot-and-chain line) and theoutput signal E_(out) (solid line). The middle value selection circuit24 always generates a signal of the middle level.

Even when the relation of the magnitude of the signals applied to theinput terminals 36A-36C changes, the middle value selection circuit 24always selects the signal of the middle level from the three inputsignals and produces its output from the output terminal 39.

The middle value selection circuit in accordance with another embodimentof the present invention will be described with reference to FIGS. 6 and7. In this embodiment, the low value selection circuits are disposed atthe prior stage and the high value selection circuit, at the posteriorstage, in constrast with the foregoing embodiment. The middle valueselection circuit 80 in this embodiment consists of the low valueselection circuits 81A through 81C and the high value selection circuit82. Each low value selection circuit consists of a pair of PNPtransistors and one constant current circuit and the high valueselection circuit 82 consists of three NPN transistors and one constantcurrent circuit.

The input terminal 36A is connected to the bases of the PNP transistors83A and 83F by a line 86. A line 87 connected to the input terminal 36Bis connected to the bases of the PNP transistors 83B and 83C. A line 88connects the input terminal 36C to the bases of the PNP transistors 83Dand 83E. The collectors of the PNP transistors 83A through 83D areconnected to one another by a line 87 which is in turn connected to theterminal 39. A line 89 connects the emitters of the PNP transistors 83Aand 83B to the constant current circuit 85A. A line 90 connects theemitters of the PNP transistors 83C and 83D to the constant currentcircuit 85B. A line 91 connected to the constant current circuit 85C isalso connected to the emitters of the PNP transistors 83E and 83F. Theconstant current circuits 85A through 85C the connected to the terminal38 by a line 92.

A line 93 connects the line 89 to the base of the NPN transistor 84A. Aline 94 connects the line 90 to the base of the NPN transistor 84B. Aline 95 connects the line 91 to the base of the NPN transistor 84C. Theemitters of the NPN transistors 84A through 84C are connected to theoutput terminal 37 by a line 96. These emitters are also connected tothe constant current circuit 85D by a line 98. A line 99 connects theconstant current circuit 85D to the line 87. On the other hand, thecollectors of the NPN transistors 84A through 84C are connected to theline 92 by the line 97.

The constant current circuit shown in FIG. 7 is also of the currentmirror type in the same way as that of the embodiment shown in FIG. 4.The constant current circuits 85A through 85C consist of PNP transistors110A through 110C, respectively. The constant current circuit 85Dconsists of an NPN transistor 111. The terminal 38 and the emitter ofthe PNP transistor 100 are connected to each other by a circuit 103. Thebase of the PNP transistor 100 is connected to the bases of the PNPtransistors 110A through 110C by a circuit 104. A line 105 having aresistor 102 connects the collector of the PNP transistor 100 to thecollector of the NPN transistor 101. The base of the NPN transistor 101is connected to that of the NPN transistor 111 by a circuit 106. Acircuit 107 connected to the terminal 39 is connected also to theemitter of the NPN transistor 101. A circuit 108 connecting the PNPtransistor 100 is connected to circuits 104 and 105. A circuit 109connecting the NPN transistor 101 is connected to circuits 105 and 106.

The operation of this middle value selection circuit 80 will bedescribed. Signals A, B and C (analog signals) having the respectivevoltage values of E₁, E₂ and E₃ are applied to the input terminals 36A,36B and 36C, respectively. It will be assumed

that the voltages have the relation E₁ >E₂ >E₃ . The signal A is appliedto the transistors 83A and 83F, the signal B, to the PNP transistors 83Band 83C and the signal C, to the PNP transistors 83D and 83E. Since thevoltages of the input signals have the relation described above, the lowvalue selection circuit 81A produces a voltage (E₂ +V_(BE)), the lowvalue selection circuit 81B produces a voltage (E₃ +V_(BE)) and the lowvalue selection circuit 81C produces a voltage (E₃ +V_(BE)).

The output signals of these low value selection circuits are applied tothe bases of the NPN transistors 84A through 84C of the high valueselection circuit 82, respectively. Since the relation (E₂ +V_(BE))>(E₃+V_(BE)) exists, the high value selection circuit 82 produces a voltagewhich is lower by the voltage V_(BE) of the NPN transistor 84A [(E₂+V_(BE) -V_(BE) ], that is, the voltage E₂, as the output. This voltageE₂ is generated at the output terminal 37.

This embodiment can provide the same effect as that of the embodimentshown in FIG. 3. Even when the current capacity of the signal to beapplied to the terminals 36A, 36B and 36C is small, it can besupplemented by the operation of the PNP transistors for amplifying theoutput current of the constant current circuit.

The embodiments shown in FIGS. 3 and 6 combine the high value sectioncircuit(s) with the low value selection circuit(s) that cause the errorshaving the inverse polarities and improve the accuracy of the middlevalue selection circuit by utilizing the characteristics of theselection circuit itself. It is not necessary to dispose afresh anydevice so as to improve the accuracy and the middle value selectioncircuit thus becomes simple in construction.

The middle value selection circuit in accordance with still anotherembodiment of the present invention will be explained with reference toFIG. 8. In the middle value selection circuit of this embodiment,resistors 115A through 115D are disposed in place of the constantcurrent circuits 33A through 33D of the middle value selection circuit24 of FIG. 3. This middle value selection circuit 112 consists of thehigh value selection circuits 113A through 113C and the low valueselection circuit 114. The high value selection circuit 113A includes aresistor 115A, which is connected to lines 47 and 50. A resistor 115B ofthe high value selection circuit 113B is connected to a line 48 and to aline 50. A resistor 115C of the high value selection circuit 113C isconnected to lines 49 and 50. A resistor 115D of the low value selectioncircuit 114 is connected to lines 56 and 57.

This circuit operates in the same way as the circuit shown in FIG. 3except that the current flowing through the high value section circuitsand the low value selection circuit is determined by the power sourceand the resistors. For this reason, the circuit is directly affected bythe influence of the power source voltage and the output voltage E_(out)of the middle value selection circuit also changes with the change inthe power source voltage.

Next, the error between the input and output signals of the middle valueselection circuit 112 will be described.

The voltage signal E_(H) produced by the high value selection circuitsis expressed by the following formula (5): ##EQU3## where β is a currentamplification ratio at the emitter ground of the transistor, and I_(b)is a current flowing through a circuit comprising the terminal 38, theresistor of the low value selection circuit, the PNP transistor of thelow value selection circuit 114, the resistors of the high valueselection circuit and the terminal 39.

The voltage signal E_(out) produced by the low value selection circuitis expressed by the following formula (7): ##EQU4## where I_(bo) is acurrent flowing through a circuit connecting the transistors andresistors of the high value selection circuit.

The output signal E_(out) of the middle value selection circuit 112 isexpressed by the following formula (9) on the basis of the formulas (5)through (8): ##EQU5## The currents I_(b) and I_(bo) are expressed by theformulas (10) and (11): ##EQU6## where V_(BB) and V_(CC) are voltages atthe terminals 38 and 39, respectively, and R is the resistance of theresistors 115A through 115D.

From the formulas (10) and (11), the currents I_(b) and I_(bo) havedifferent values. Accordingly, the second item of the right side of theformula (9) does not become zero so that the voltage E_(out) does notbecome equal to the voltage E₂. Though the error becomes greater thanthat in the embodiment shown in FIG. 3, the error of one of the high andlow value selection circuits is compensated for by the error of theother because the errors of these selection circuits have the oppositepolarities. Accordingly, this embodiment can improve the accuracy overthe prior art circuit.

The foregoing embodiments relate to the middle value selection circuitsusing the transistors but the middle value selection circuit can beconstructed using diodes which are unidirectionally conductive devices.Such an embodiment will be described with reference to FIG. 9.

The middle value selection circuit 116 consists of the high valueselection circuits 117A-117C and the low value selection circuit 118.The high value selection circuit 117A consists of a pair of diodes 119A,119B and the constant current circuit 160A. The high value selectioncircuit 117B consists of a pair of diodes 119C, 119D and the constantcurrent circuit 160B. The high value selection circuit 117C consists ofa pair of diodes 119E, 119F and the constant current circuit 160C. Thehigh value selection circuit 118 consists of three diodes 119G-119I andthe constant current circuit 33D. The diodes of the high value selectioncircuits are complementary to those of the low value selection circuit.This also holds true of the embodiment shown in FIG. 11.

The input terminal 36A is connected to the anodes of the diodes 119A and119F by a line 120. The input terminal 36B is connected to the anodes ofthe diodes 119B and 119C by a line 121. A line 122 connects the inputterminal 36C to the anodes of the diodes 119D and 119E. A line 123connects the cathodes of the diodes 119A and 119B to the constantcurrent circuit 160A. A line 124 connected to the cathodes of the diodes119C and 119D is connected to the constant current circuit 160B. A line125 connects the constant current circuit 160C to the cathodes of thediodes 119E and 119F. The constant current circuits 160A through 160Care connected to the terminal 39 by a line 126. The line 123 and thecathode of the diode 191I are connected to each other by a line 127. Aline 128 connects the line 124 to the cathode of the diode 119H. A line129 connected to the line 125 is connected to the cathode of the diode119G. A line 130 connected to the anodes of the diodes 119G through 119Iis connected to the output terminal 37. The terminal 38 and the constantcurrent circuit 33D are connected to each other by a line 131. A line132 connected to the constant current circuit 33D is connected to theanodes of the diodes 119G through 119I.

Signals A, B and C (analog signals) having voltages E₁, E₂ and E₃satisfying the relation E₁ >E₂ >E₃, respectively, are applied to theinput terminals 36A, 36B and 36C, respectively. The signal A is appliedto the anodes of the diodes 119A and 119F, the signal B, to the anodesof the diodes 119B and 119C and the signal C, to the anodes of thediodes 119D and 119E. The high value selection circuit 117A produced avoltage [E₁ +(-V_(F))], the high value selection circuit 117B produces avoltage [E₂ +(-V_(F))] and the high value selection circuit 117Cproduces a voltage [E₃ +(-V_(F))]. Here, (-V_(F)) represents the voltagedrop of the diode in the forward direction. The output voltage of eachhigh value selection circuit is applied to the cathodes of the diodes119G through 119I of the low value selection circuit 118. The low valueselection circuit 118 produces a voltage which is higher by the forwardvoltage V_(F) of the diode, that is, a voltage [ (E₂ +V_(F))+V_(F) ]=E₂,from the output terminal 37.

The constant current circuits 160A through 160C and 33D are currentmirror type circuits shown in FIG. 10. The constant current circuits160A through 160C disposed in the high value selection circuits 117Athrough 117C produce a current which is by twice higher than the currentproduced by the constant current circuit 33D of the low value selectioncircuit 118. These constant current circuits 160A through 160C and 33Dwill be explained with reference to FIG. 10. In the drawing, likereference numerals are used to identify like constituents as in FIG. 4.

The constant current circuits are different from the constant currentcircuit shown in FIG. 4 in that the constant current circuits 160Athrough 160C each consist of two transistors. In other words, theconstant current circuit 160A consists of NPN transistors 68A and 68B,the constant current circuit 160B consists of NPN transistors 69A and69B and the constant current circuit 160C consists of NPN transistors70A and 70B. The bases of these six transistors are connected to theline 64. The collectors of the NPN transistors 68A and 68B are connectedto the line 123 and their emitters, to the line 126. The collectors ofthe NPN transistors 69A and 69B are connected to the line 124 and theiremitters, to the line 126. The collectors of the NPN transistors 70A and70B are connected to the line 125 and their emitters, to the line 126.

If these constant current circuits are employed, the current value ofthe constant current circuits of the high value selection circuitbecomes twice that of the constant current circuit of the low valueselection circuit. In other words, the current produced from theconstant current circuits 160A through 160C is 2I_(o) and the currentproduced from the constant current circuit 33D becomes I_(o).Accordingly, the currents flowing through the diodes 119A through 119Fcan be made virtually equal to one another and the voltage drops of thediodes 119A through 119F in the forward direction can be made equal. Thevoltage -V_(F), which is the error of the high value selection circuits117A through 117C, can be compensated for by the error V_(F) of the lowvalue selection circuit 118.

In the middle value selection circuits using the diodes, the middlevalue can be obtained with a high level of accuracy in the same way asin the embodiment shown in FIG. 3 provided that the values of the threeinput signals are all different such as when E₁ >E₂ >E₃, for example.However, if at least two of the three input signals are equal to eachother such as when E₁ =E₂ >E₃, for example, the output signal of themiddle value selection circuit 116 does not become equal to the middlevalue of these three input signals. In contrast, in the embodiment shownin FIG. 3, the middle value of the three input signals of the middlevalue selection circuit 24 becomes equal to its output signal even whenE₁ =E₂ >E₃. Since this embodiment uses the diodes for the high and lowvalue selection circuits, the number of the necessary components can bereduced and the construction of the middle value selection circuit canbe remarkably simplified.

When the diodes are used, the low value selection circuit can bedisposed at the prior stage with the high value selection circuits atthe posterior stage, in the same way as in the embodiment shown in FIG.6. Such an embodiment is illustrated in FIG. 11. The middle valueselection circuit 133 of this embodiment consists of the low valueselection circuits 134A through 134C and the high value selectioncircuit 135. The low value selection circuit 134A consists of the diodes119A, 119B and the constant current circuit 161A. The low valueselection circuit 134B consists of the diodes 119C, 119D and theconstant current circuit 161B and the low value selection circuit 134Cconsists of the diodes 119E, 119F and the constant current circuit 161C.

These constant current circuits are of the current mirror type as shownin FIG. 12. The current produced from the constant current circuits 161Athrough 161C of the low value selection circuits 134A through 134C is2I_(o) while the current produced from the constant current circuit 85Dof the high value selection circuit 135 is I_(o). Hence, the currentvalue of the former is twice that of the latter. The constant currentcircuits 161A through 161C consist of parallel circuits of PNPtransistors 110A-156A, 110B-156B and 110C-156C, respectively. The basesof the PNP transistors 110A through 110C and 156A through 156C areconnected to a circuit 104. The other constructions in FIG. 12 are thesame as those of the constant current circuit shown in FIG. 7.

The lines 120, 121 and 122 in the middle value selection circuit 133 areconnected to the cathodes of the diodes 119A through 119F, respectively.Lines 123, 124 and 125 connected to the constant current circuits 161Athrough 161C are connected to the anodes of the diodes 119A through119F, respectively. A line 126 connects the terminal 38 to the constantcurrent circuits 161A through 161C. Lines 127, 128 and 129 are connectedto the anodes of the diodes 119I, 119H and 119G of the high valueselection circuit 135, respectively. A line 130 is connected to thecathodes of the diodes 119G, 119H and 119I. Lines 131 and 132 connectedto the constant current circuit 85D are connected to the terminal 39 andto the cathodes of the diodes 119G through 119I, respectively.

Signals A, B and C are applied to the input terminals 36A, 36B and 36C,respectively. These signals A, B and C (analog signals) have thevoltages E₁, E₂ and E₃ that satisfy the relation E₁ >E₂ >E₃ in the sameway as in the embodiment shown in FIG. 9. The low value selectioncircuit 134A produces a voltage (E₂ +V_(F)). The low value selectioncircuit 135B produces a voltage (E₃ +V_(F)) and the low value selectioncircuit 134C does a voltage (E₃ +V_(F)). In other words, each low valueselection circuit produces the voltage which is higher by the forwardvoltage of each diode than the voltage of the input signal. The outputvoltage of each low value selection circuit is applied to the anode ofeach diode of the high value selection circuit 135. The high valueselection circuit 135 produces a voltage E₂ which is lower by thevoltage drop (-V_(F)) in the forward direction of the diode 119I thanthe voltage (E₂ +V_(F)). This embodiment provides the same effect asthat of the embodiment shown in FIG. 9.

The middle value selection circuit 165 in accordance with still anotherembodiment of the present invention is shown in FIG. 13. The middlevalue selection circuit 165 consists of the high value selectioncircuits 166A through 166C and the low value selection circuit 118. Themiddle value selection circuit 165 is formed by replacing the diodes ofeach high value selection circuit of the middle value selection circuit116 shown in FIG. 9 by an NPN transistor. The high value selectioncircuit 166A consists of a pair of NPN transistors 31A, 31B and oneconstant current circuit 160A. The high value selection circuit 166Bconsists of a pair of NPN transistors 31C, 31D and one constant currentcircuit 160B. The high value selection circuit 166C consists of a pairof NPN transistors 31E, 31F and one constant current circuit 160C. TheNPN transistors of the high value selection circuit of this embodimentare complementary to the diodes of the low value selection circuit. Eachconstant current circuit has the same construction as that shown in FIG.10.

This embodiment provides the same effect as that of the embodiment shownin FIG. 9 except that its construction becomes a bit more complicatedbecause the NPN transistors are used. Since this embodiment uses thediodes for the low value selection circuit, the construction can be moresimplified when conpared with the construction of the embodiment shownin FIG. 3. Since the NPN transistors are used for the high valueselection circuits, the output current of the constant current circuitscan be supplemented by the current amplification operation of the NPNtransistors even if the current capacity of the input signals applied tothe terminals 36A, 36B and 36C is small.

FIG. 14 shows the middle value selection circuit 167 in accordance withstill another embodiment of the present invention. In this embodiment,the diodes of the low value selection circuits of the middle valueselection circuit 133 shown in FIG. 11 are replaced by the PNPtransistors. The low value selection circuit 168A consists of a pair ofPNP transistors 83A, 83B and one constant current circuit 161A. The lowvalue selection circuit 168B consists of a pair of PNP transistors 83C,83D and one constant current circuit 161B. The low value selectioncircuit 168C consists of a pair of PNP transistors 83E, 83F and oneconstant current circuit 161C. Each constant current circuit has thesame as one shown in FIG. 12. The PNP transistors of the low valueselection circuits of this embodiment are complementary to the diodes ofthe high value selection circuit. This embodiment can provide the sameeffect as that of the middle value selection circuit 165 shown in FIG.13. In this embodiment, too, the output current of the constant currentcircuit can be supplemented by the current amplification operation ofthe PNP transistors even if the current capacity of the input signalsapplied to the terminals 36A, 36B and 36C is small, in the same way asthe middle value selection circuit 165.

If the high and low value selection circuits of the middle valueselection circuit 24 shown in FIG. 3 are composed of NPN and PNPtransistors produced with the same characteristics, the voltage V_(BE)of the NPN transistors becomes completely equal to the voltage V_(BE) ofthe PNP transistors, as described previously. Accordingly, the error ofthe high value selection circuit and the error of the low valueselection circuit can be completely cancelled by each other. If themiddle value section circuit 24 is composed of an integrated circuit(hereinafter referred to as "IC"), however, the absolute values of thevoltages V_(BE) of the NPN and PNP transistors are likely to becomedifferent. In order to make equal the absolute values of the voltagesV_(BE) of the NPN and PNP transistors formed on the same substrate toeach other, the production steps of the IC of the middle value selectioncircuit 24 becomes remarkably complicated. When the NPN and PNPtransistors are formed on the same substrate, therefore, theconstruction of these transistors is simplified in order to simplify theproduction steps. If the transistors are of the simplified type,however, the absolute values of the voltages V_(BE) of the transistorsbecome different. In such a case, the middle value selection circuit cannot be mass-produced. The inventors of the present invention haveexamined the structures of the IC of the middle value selection circuitsin which the errors of the high and low value selection circuits can bemutually compensated for and moreover, the production steps can besimplified. As a result, the middle value selection circuit shown inFIGS. 15 and 16 can be obtained.

The middle value selection circuit 169 of the embodiment of theinvention shown in FIG. 15 is formed by replacing the diodes of the lowvalue selection circuit of the middle value selection circuit 165 shownin FIG. 13 by NPN transistors and by connecting the collectors to basesof these NPN transistors so as to furnish them with the diode function.Each NPN transistor whose collector is connected to the base is aunidirectionally conductive device.

The middle value selection circuit 169 consists of the high valueselection circuits 166A through 166C and the low value selection circuit170. The low value selection circuit 170 consists of three NPNtransistors 32A-32C and the constant current circuit 33D. The constantcurrent circuit of the middle value selection circuit 169 is shown inFIG. 10. The emitter of the NPN transistor 32A is connected to a line129 and the emitter of the transistor 32B is connected to a line 128.The emitter of the NPN transistor 32C is connected to a line 127. Acircuit 130 is connected to the bases of the NPN transistors 32A-32C.The collectors of these NPN transistors are connected to a line 130 vialines 171A, 171B and 171C, respectively. The other construction of thisembodiment is the same as that of the embodiment shown in FIG. 13. TheNPN transistors of the high value selection circuits 166A-166C arecomplementary to the NPN transistors of the low value selection circuit170 whose bases are connected to the emitters and which thus function asthe diodes.

This embodiment uses the NPN transistors which are of the same kind inboth high and low value selection circuits 166A-166C and 170.Accordingly, the IC of the middle value selection circuit 169 can beobtained by simple fabrication steps. Moreover, the characteristics ofthe semiconductor devices of the high value selection circuit in theresulting middle value selection circuit 169 can be made equal to thoseof the semiconductor devices of the low value selection circuit. Inother words, the absolute value of the voltage V_(BE) of the NPNtransistors of the high value selection circuit becomes equal to that ofthe NPN transistors (or voltage V_(F) in terms of diodes) of the lowvalue selection circuit 170. Since the middle value selection circuit ofthis embodiment can simplify the production steps and can cancelmutually the errors of the high and low value selection circuits, it isparticularly suitable for mass-production.

The constant current circuit in this embodiment is of the current mirrortype as shown in FIG. 10. Accordingly, the current value of the constantcurrent circuits of the high value selection circuits 166A-166C becomes2I_(o) and twice the current value I_(o) of the low value selectioncircuit 170.

The middle value selection circuit in accordance with still anotherembodiment of the present invention shown in FIG. 16 uses the PNPtransistors of the low value selection circuit at the prior stage of themiddle value selection circuit 167 shown in FIG. 14 as the semiconductordevices of the high value selection circuit at the posterior stage. Thisembodiment can solve the problem encountered during production of the ICof the middle value selection circuit, in the same way as the embodimentshown in FIG. 15.

The middle value selection circuit 172 consists of the low valueselection circuits 168A-168C and the high value selection circuit 173.The current mirror type constant current circuit shown in FIG. 12 isused as the constant current circuit for each of the high and low valueselection circuits. The high value selection circuit 173 consists of thePNP transistors 84A-84C and the constant current circuit 85D. The basesof the PNP transistors 84A-84C are all connected to a line 130. Thisline is connected to the collectors of the PNP transistors 84A-84C bylines 174A-174C, respectively. The emitter of the PNP transistor 84A isconnected to a line 127 and the emitter of the PNP transistor 84B, to aline 128. The emitter of the PNP transistor 84C is connected to a line129.

The PNP transistors 84A-84C of the high value selection circuit 173function as the diodes. The PNP transistors of the low value selectioncircuits 168A-168C are complementary to the PNP transistors functioningas the diodes, of the high value selection circuit 173. The PNPtransistors of this embodiment whose collectors are connected to thebases are unidirectionally conductive devices. This embodiment canprovide the same effect as the embodiment shown in FIG. 15.

The constant current circuit shown in each of FIGS. 9, 11 and 13 through16 may be formed by using the resistors as in the embodiment shown inFIG. 8. In such a case, the value of the resistors of the threeselection circuits disposed at the prior stage is made 1/2 of theresistance of the resistors of one selection circuit disposed at theposterior stage so that the current value produced from the resistors ofthe selection circuits of the prior stage becomes twice the currentvalue produced from the resistors of the selection circuit of theposterior stage.

The foregoing embodiments illustrate the middle value selection circuitswhen the input signals of the voltage type. FIG. 17 shows the middlevalue selection circuit when the input signals are of the current type.The middle value selection circuit 136 consists of the voltage typemiddle value selection circuit shown in FIG. 3, a current voltageconversion circuit 137 and a voltage-current conversion circuit 147. Inother words, the middle value selection circuit 136 converts the currentsignal to the voltage signal, applies it to the voltage type middlevalue selection circuit and then converts the voltage signal, which isthe output signal of the voltage type middle value selection circuit, tothe current signal as its output.

The current-voltage conversion circuit 137 consists of three lightisolators each of which consists in turn of a light emission diode as alight emitting element and a photo transistor as a light receivingelement, and three resistors. The first light isolator consists of thelight emission diode 138A and the photo transistor 139A, the secondconsists of the light emission diode 138B and the photo diode 139B andthe third consists of the light emission diode 138C and the phototransistor 139C. The photo transistors 139A-139C are of the NPNtransistor type. The anode of the light emission diode 138A is connectedto a terminal 143A, the cathode of the light emission diode 38A isconnected to a terminal 143B and a terminal 144A is connected to theanode of the light emission diode 138B. A terminal 144B is connected tothe cathode of the light emission diode 138B. A terminal 145A isconnected to the anode of the light emission diode 138C and a terminal145B is connected to the cathode of the light emission diode 138C. Theterminals 143A, 144A and 145A are connected to the sensors 21, 22 and 23shown in FIG. 2 (each of said sensors being grounded). The terminals143B, 144B and 145B are grounded. The emitters of the photo transistors139A, 139B and 139C are grounded via resistors 140A, 140B and 140C,respectively. The input terminals 36A, 36B and 36C of the middle valueselection circuit 24 are connected to the emitters of the phototransistors 139A, 139B and 139C via lines 146A, 146B and 146C,respectively. A power terminal 141 for d.c. biasing is connected to thecollectors of the photo transistors 139A through 139C by a line 142.

The voltage-current conversion circuit 147 consists of an NPN transistor148 and two resistors 149 and 152. The base of the NPN transistor 148 isconnected to the output terminal 37 of the middle value selectioncircuit 24 by a line 150. The emitter of the NPN transistor 148 isgrounded via a resistor 149. A line 153 connects a terminal 155A to thecollector of the NPN transistor 148. A power terminal 151 for d.c.biasing is connected to a terminal 155B by a line 154 having theresistor 152. Terminals 155A and 155B are connected to a load inside thecontroller.

It will be assumed that a current signal I₁ flowing through theterminals 143A and 143B, a current signal I₂ flowing through theterminals 144A and 144B and a current signal I₃ flowing through theterminals 145A and 145B are applied to the first, second and third lightconverters, and that the current signals (analog signals) have therelation I₁ >I₂ >I₃. The photo transistors 139A-139C of these lightconverters produce current signals KI₁, KI₂ and KI₃, respectively. Sincethe resistance values of the resistors 140A through 140C are equal toone another, the current-voltage circuit 137 produces voltage signalsE₁, E₂ and E₃ that are proportional to the input currents at the inputterminals 36A, 36B and 36C, respectively.

Upon receiving these voltage signals E₁, E₂ and E₃, the middle valueselection circuit 24 produces the voltage signal E₂ of the middle levelas described previously.

The voltage-current conversion circuit 147 converts the input voltagesignal E₂ to the current signal I₂ by the operation of the NPNtransistor 148 and the resistors 149 and 152. This current signal I₂ isproduced at the terminal 155A.

This embodiment provides the same effects as the embodiment shown inFIG. 3 and makes it easy to select the middle value of the currentsignals. Since the light converters are used at the input stage,electric insulation can be established easily. Accordingly, even when ahigh current is detected by a detector, not only the middle valueselection circuit but also the controller disposed at the posteriorstage with respect to the former can be protected.

Any one of the aforementioned middle value selection circuits 80, 112,116 and 133 can be used in place of the middle value selection circuit24 of the middle value selection circuit 136.

When the current signal as the input signal is negative, the middlevalue selection circuit 136 may be arranged in such a fashion that thephoto transistors forming the light converters of the middle valueselection circuit 136 are changed to the PNP type, the polarity of thepower source at the power terminals 141 and 151 is changed and the NPNtransistor 148 is changed to the PNP transistor.

In accordance with the present invention that has been described in theforegoing, the errors occurring in both high and low value selectioncircuits can be mutually compensated for and the middle value selectioncircuit having high accuracy and yet being simple in construction can beobtained.

What is claimed is:
 1. In a middle value selection circuit whichconsists essentially of three first selection means for receivingmutually different combinations of two analog signals among three inputanalog signals and selecting and producing either one of the greater orsmaller signal of said two analog signals and second selection means forreceiving as an input signal the output signals of said first selectionmeans to produce a signal representative of a minimum value of the inputsignals when said first selection means selects said greater signals anda signal representative of a maximum value of the input signals whensaid first selection means selects said smaller signals, the improvementwherein said first and second selection means are composed ofsemiconductor devices and there is further provided constant currentmeans for supplying a constant current for said first and secondselection means so that an error component resulting from thesemiconductor devices of said first selection means and included in thesignal from said first selection means is complementary to an errorcomponent caused by the semiconductor device of said second selectionmeans and said error components have stable values in spite ofvariations in the values of said input signal, wherein each of saidfirst selection means consists of a pair of semiconductor devices in theform of transistors of a first conductivity type and a first constantcurrent circuit connected in common to said pair of semiconductordevices, and said second selection means consists of three semiconductordevices in the form of unidirectionally conductive elements of a secondconductivity type and a second constant current circuit connected toeach of said semiconductor devices of said second conductivity type,said first and second conductivity types being complementary to eachother said first constant current circuit producing a current whosemagnitude is twice the magnitude of the output current of said secondconstant current circuit.
 2. The middle value selection circuit asdefined in claim 1 wherein said unidirectionally conductive device is adiode.
 3. The middle value selection circuit as defined in claim 1wherein said unidirectionally conductive element is a transistor, twoelectrodes of which are connected to each other so that the transistoroperates as a unidirectionally conductive element and the remainingelectrode is supplied with the output of said first selection means. 4.The middle value selection circuit as defined in claim 3 wherein thebases of said pair of transistors in each of said first selection meansare connected to receive said two analog signals of the differentcombination, one of the remaining two electrodes thereof being commonlyconnected to a supply of power, and the other electrodes thereof beingconnected in common to said first constant current source and to one ofthe electrodes of said transistor of said second selection means, andthe remaining two electrodes of said transistor of said second selectionmeans is connected to the output terminal of said second selection meansand to said second constant current circuit.
 5. In a middle valueselection circuit which consists essentially of three first selectionmeans for receiving mutually different combinations of two analogsignals among three input analog signals and selecting and producingeither one of the greater or smaller signal of said two analog signalsand second selection means for receiving as an input signal the outputsignals of said first selection means to produce a signal representativeof a minimum value of the input signals when said first selection meansselects greater signals and a signal representative of a maximum valueof the input signals when said first selection means selects saidsmaller signals, the improvement wherein said first and second selectionmeans are composed of semiconductor devices and there is furtherprovided constant current means for supplying a constant current forsaid first and second selection means so that an error componentresulting from the semiconductor devices of said first selection meansand included in the signal from said first selection means iscomplementary to an error component caused by the semiconductor deviceof said second selection means and said error components have stablevalues in spite of variations in the values of said input signal,wherein each first selection means includes a pair of semiconductordevices and a first constant current circuit and said second selectionmeans includes three semiconductor devices and a second constant currentcircuit, each of said semiconductor devices in said second selectionmeans being unidirectionally conductive elements and said first constantcurrent circuit producing a current whose magnitude is twice themagnitude of the output current of said second constant current circuit.6. The middle value selection circuit as defined in claim 5 wherein saidunidirectionally conductive elements are diodes.
 7. The middle valueselection circuit as defined in claim 5 wherein said unidirectionallyconductive element is a transistor, two electrodes of which areconnected to each other so that the transistor operates as aunidirectionally conductive element and the remaining electrode issupplied with the output of said first selection means.
 8. The middlevalue selection circuit as defined in claim 5 wherein said semiconductordevices in each of said first selection means are unidirectionallyconductive elements.
 9. The middle value selection circuit as defined inclaim 8 wherein said semiconductor devices in each of said firstselection means are diodes.
 10. The middle value selection circuit asdefined in claim 8 wherein the unidirectional elements of said first andsecond selection means are diodes, each of said first selection meansbeing composed of a pair of the diodes connected in the electricallyopposite direction to each other and said first constant currentcircuit, and said second selection means being composed of three diodesconnected to the corresponding one of said pairs of diodes and saidsecond constant current circuit.
 11. In a middle value selection circuitwhich consists essentially of three first selection means for receivingmutually different combinations of two analog signals among three inputanalog signals and selecting and producing either one of the greater orsmaller signal of said two analog signals and second selection means forreceiving as an input signal the output signals of said first selectionmeans to produce a signal representative of a minimum vlaue of the inputsignals when said first selection means selects said greater signals anda signal representative of a maximum value of the input signals whensaid first selection means selects said smaller signals, the improvementwherein said first and second selection means are composed ofsemiconductor devices and there is further provided constant currentmeans for supplying a constant current for said first and secondselection means so that an error component resulting from thesemiconductor devices of said first selection means and included in thesignal from said first selection means is complementary to an errorcomponent caused by the semiconductor device of said second selectionmeans and said error components have stable values in spite ofvariations in the values of said input signal, wherein each of saidfirst selection means consists of a pair of semiconductor devices of afirst conductivity type and a first constant current circuit connectedin common to said pair of semiconductor devices, and said secondselection means consists of three semiconductor devices in the form oftransistors of a second conductivity type and a second constant currentcircuit connected to each of said semiconductor devices of said secondconductivity type, said first and second conductivity types beingcomplementary to each other, wherein said semiconductor devices of saidfirst selection means are transistors and two electrodes of eachtransistor of said first selection means being connected to each otherso that the transistor operates as a unidirectionally conductive elementand the remaining electrode is connected to receive said analog signalsand said first constant current circuit produces a current twice theoutput current of said second constant current circuit.
 12. In a middlevalue selection circuit which consists essentially of three firstselection means for receiving mutually different combinations of twoanalog signals among three input analog signals and selecting andproducing either one of the greater or smaller signal of said two analogsignals and second selection means for receiving as an input signal theoutput signals of said first selection means to produce a signalrepresentative of a minimum value of the input signals when said firstselection means selects said greater signals and a signal representativeof a maximum value of the input signals when said first selection meansselects said smaller signals, the improvement wherein said first andsecond selection means are composed of semiconductor devices and thereis further provided constant current means for supplying a constantcurrent for said first and second selection means so that an errorcomponent resulting from the semiconducor devices of said firstselection means and included in the signal from said first selectionmeans is complementary to an error component caused by the semiconductordevice of said second selection means and said error components havestable values in spite of variations in the values of said input signal,wherein the improvement further comprises first conversion means forconverting said analog signals as a current signal into voltage analogsignals and for supplying said voltage analog signal to said firstselection means, and second conversion means for converting the outputsignal, as a voltage signal, of said second selection means into acurrent signal.
 13. The middle value selection circuit as defined inclaim 12 wherein said first conversion means is a light converter whichhas a photodiode converting a current input signal into a light signaland a photo transistor changing the light signal to a voltage signal.